During low-speed operation of light-duty passenger vehicle engines, moving metal surfaces in the engine and valvetrain assemblies are typically exposed to boundary- and mixed-layer lubrication contacts. In these lubrication regimes, the lubricant film thickness is insufficient to adequately separate metal surfaces, and microscale asperities come into contact which causes plastic deformation (wear) to occur. Thus, to protect against wear under low-speed conditions, lubricants are formulated to contain thermomechanically-activated species which deposit on or react with steel surfaces. The formed layer, or tribofilm, acts as a barrier to prevent metal-metal contact and potential wear. Thick tribofilms prevent contact between even the highest asperities in rough regions.
In general, tribofilms are sacrificial layers, with material being removed as surfaces come into contact and then rebuilt as re-lubrication of the contact delivers more of the film-forming components to the interface. Successful wear protection is thus enabled by rates of film generation that are on par with the rate of film removal. A significant challenge relates to the ability of a tribofilm to remain adequately thick under severe contact conditions where interface re-lubrication may not be guaranteed.
This issue is particularly relevant with the development of the Ford chain wear (FCW) engine test in the new ILSAC GF-6 category. In this test, the rated parameter is wear on the timing chain. The metal-metal contact between the link pin and link plate is a small-displacement reciprocating motion, causing oil to be squeezed out of the contact with challenged replenishment. Furthermore, the FCW test is run in a gasoline direct-injection (GDI) engine. This fuel delivery configuration produces some amount of soot content that may compete with oil additives for the surface and/or increase local oil viscosity in narrow contact regions through chemical or physical means. Identifying formulation strategies which provide thick-forming films that provide adequate separation of surfaces even in challenged lubrication conditions is thus of great importance for reducing wear.
Zinc dialkyldithiophosphates (ZDDPs) are considered to be the main contributor to tribofilm formation. Unfortunately, ZDDPs release volatile phosphorus as a decomposition product which can poison catalytic converters, and thus strict limits on phosphorus and phosphorus volatility have been established for modern engine oil specifications. Limiting the amount of ZDDP in a formulation removes a main lever for increasing tribofilm thickness.
A major challenge in engine oil formulation is achieving wear protection in the FCW test, in particular, developing lubricating oil formulations which provide thick-forming tribofilms that provide adequate separation of surfaces even in challenged lubrication conditions.